(1) Field of the Invention
The present invention relates to a solid propellant power plant having a unitary mass of propellant ignited to create a hot gas which may be the moving force of a reaction motor; more particularly, the invention relates to such a mass having an internal surface of a specific configuration to control the burning rate of the propellant. The invention further relates to such a mass wherein the specific configuration may be helically grooved and conical and be formed by a conforming article shaping apparatus.
(2) Description of the Related Art
For the purposes of the present application, a grain of solid rocket propellant is defined as a unitary mass having a particular form or as plurality of individual forms united to define a particular composite form, the form having a surface of specific configuration. Such a grain is thus not a granule used in combination with other unconnected granules in particulate or powdered material.
It is known to provide such a grain of solid rocket propellant with a cavity configured to vary or otherwise control the burning rate of the propellant. For examples:
U.S. Pat. No. 4,015,427 issued Apr. 5, 1977 to Brooks discloses such a grain for use in a spherical casing and having an internal cavity defined by a plurality of regressively burning star point segments and a plurality of segments with parallel side walls positioned there between.
The segments are defined by planes parallel to the axis of a nozzle attached to the casing, and the segments are configured to provide a sharp transition from boost to sustain operation.
U.S. Pat. No. 4,578,947 issued Apr. 1, 1986 to Hodges, Jr. et al discloses a solid propellant grain with a central, solid, cylindrical rod portion extending axially of a nozzle and having a diameter of xe2x80x9c2Rxe2x80x9d. To provide neutral burning, the rod portion is disposed coaxially within a hollow, cylindrical web portion having a radial thickness of xe2x80x9cRxe2x80x9d.
U.S. Pat. No. 5,111,657 issued May 12, 1992 to Hivert et al. discloses such a grain or block of propellant which is coaxially related to a nozzle, is externally cylindrical, and defines a longitudinal central duct. The block has a cylindrical, downstream portion smaller in diameter than two thirds of the diameter of the throat of the nozzle and has a cylindrical, larger diameter, upstream portion connected to the downstream portion by a portion in the shape of a truncated cone. The length of the upstream portion is between two and four times the external diameter of the block and less than the length of the downstream portion, and the total length of the block is more than six times its external diameter.
It is also known to provide helical grooves about a conical opening in a conically threaded closure system as disclosed in U.S. Pat. No. 5,785,195 issued Jul. 28, 1998 to Zwemer et al.
In addition to providing desired burn rates or variations in burn rates during the combustion of a grain of solid rocket propellant by the surface configuration of the grain, it is desirable that the configuration of a cavity of such a grain be adapted to a variety of casing shapes, waste a minimal amount of the volume within the casing, and be convenient to manufacture, as by casting.
It may also be desirable that the cavity configuration provide a large initial burning surface area together with a neutral burning rate; and, with pressure-sensitive propellants, it is desirable that the area of a port through which combustion products exit from the grain be relatively large in relation to the area of the throat of an associated nozzle.
To maintain the desired burning characteristics, it is important that the grain not be fissured or otherwise have its surface disrupted by strains due to burning or vibration. In this regard, it has been found that a cavity with surfaces extended along the axis of the cavity, as in the typical star or grooved shaped cavity, results in longitudinal strains in the propellant.
Finally, it is desirable that the configuration of the cavity of a grain of solid propellant not, as was common in such configurations of the prior art, provide certain of the above-mentioned desirable features by sacrificing others of these features.
A propellant grain configured for reception in a cylindrical casing has a central cavity extended generally along the casing axis. The cavity has a central opening which may be cylindrical or be a conical frustum and, at one end serves as port for exit of combustion products from the grain. The grain is characterized by a plurality of surfaces extending circumferentially about this opening and disposed normally or helically in relation to the casing axis so as to define, between the surfaces, alternating sections of propellant and a groove or grooves in the propellant.
If the surfaces are helical, they may define a single groove or multiple grooves. The grooves extend generally radially from and open into the opening. The radial extent of the grooves may be selected to provide the grain with a large surface area for initial burning, while the grooves may be narrow axially to provide a high propellant volume in the casing, and the port area may be selected to be relatively large in relation to an adjacent nozzle attached to casing. The grain may be made substantially neutral burning by selecting the distance from the radial ends of the groove sections to the casing to be about twice the distance between axially adjacent groove sections. The groove is preferably helical to relieve and distribute longitudinal strains in the grain.
Such a central cavity including a helical groove may be formed by casting the propellant into the casing in which is disposed a conforming mandrel. The mandrel has a central core bearing a helical ridge and is thus removable from the solidified grain by rotating the mandrel to xe2x80x9cunscrewxe2x80x9d the mandrel from the grain. For better release and removal of the mandrel from the grain, the cavity may be constructed with its central opening and the periphery of the groove being defined by concentric conical frustums, with the groove narrowing in direction axially of the central opening as the groove recedes radially from the opening, and with the groove narrowing progressively axially of the central opening as the groove proceeds along the opening.
An object of the present invention is to provide a propellant grain configuration having a high propellant volume together with a large initial surface area.
Another object is to provide such a configuration wherein longitudinal strains are minimized and are distributed more tangentially along the grain.
A further object is to provide such a configuration that may be configured for a large port area and for either desired burning rate variations or for neutral burning.
Yet another object is to provide such a configuration that is convenient to manufacture.
A further object is to provide such a configuration having the foregoing advantages and fully effective for propulsion.